The European Commission said it would "take some time" for the bloc to agree its position.
The timeline set by the European Union could add to pressure for an extension to the negotiating period but Prime Minister Boris Johnson has been adamant that a deal will be reached by the end of December.
European Commission spokesman Eric Mamer said the process of agreeing the EU's position could only begin after Brexit, meaning trade talks could begin as late as in March.
"This, we know, will take some time, which is why we have said we will start negotiations as quickly as we can, but it will certainly not be before the end of February, beginning of March," he said.
"This is not a slowing down or speeding up of the process.
"This is simply the nature of the institutional process and the consultations that need to take place before the negotiation directives can be formally adopted."
Talks between the UK and US could begin before negotiations with Brussels.
Both the UK and US say "extensive preparations" have been made, though no timetable has been set.
Donald Trump has repeatedly spoken of his desire to reach an agreement with Mr Johnson, and the timing of the US presidential election means that a deal in the summer of 2020 could be his goal.
The Prime Minister's official spokesman said: "We are free to begin discussions with countries around the world from February 1. We are ready to begin discussions with the EU from February 1.
"The EU have various processes to go through before they are ready to sit down and have those discussions with us."
The UK remains committed to agreeing a deal with Brussels by the end of the year.
Speaking to BBC Breakfast's Dan Walker last week, the Prime Minister said it was "enormously likely, epically likely" that a trade deal will be reached.
But the EU repeated its warning that the UK's plans to diverge from the Brussels rulebook would limit access to the bloc's markets.
The new European Commission president, Ursula von der Leyen, warned earlier this month that it would be "impossible" to reach a comprehensive trade deal by the end of 2020.
Mr Mamer added: "There is a link between moving away from EU regulations and the degree of access that is possible into the single market."
The Prime Minister's spokesman said the new situation would mean "it will be the UK which determines its own rules and laws".
"It will not be a ruletaker from Brussels or the EU's institutions."